Overview

Galway Bay is a large bay located on the west coast of Ireland bordered by Co. Clare to the south and Co. Galway to the north. The bay is 62 kilometres long from the Brannock Islands in the west to Oranmore in the east. From Doolin to Lettermullan the mouth of the bay is 33 kilometres wide, but narrows at Black Head to 10 kilometres.

The bay is dominated and protected by the Aran Islands. The largest of which is Inishmore to the north, Inisheer to the south and Inishmaan between them. The main rivers entering the bay are the River Corrib at Galway and the Owenboliskey at an Spidéal. Along the south coast of the bay there are many minor freshwater inputs as well as submarine freshwater sources due to the drainage patterns associated with the karst landforms of the Burren.

INFOMAR Survey History

INFOMAR surveys of Galway Bay began in 2006 when Tenix LADS undertook a lidar survey of an area along the southern coast of the bay from Oranmore Bay to Black Head. This was followed by a survey on the Celtic Voyager which achieved exceptional success, covering a large area from the Aran Islands east beyond Black Head.

In 2007, IMAR Surveys Ltd. were contracted to undertake a hydrographic and geophysical survey of the inner part of Galway Bay. In 2008, another lidar survey was completed along the northern coast of the Bay and the eastern margin of the Aran Islands. Later in the year, the final gaps between lidar and earlier multibeam coverage were filled by the Celtic Voyager in the south of the bay off the Clare coast and along the eastern coast of the Aran Islands. This produced a seamless merging of previously acquired data into one comprehensive dataset.

Lidar surveys were carried out in Kilkieran & Bertraghboy Bays in 2010.

Surveys were carried out by the Celtic Voyager for the the Sustainable Energy Authority of Ireland (SEAI) south of the Aran Islands off the Clare Coast between 2012 and 2014.

Map showing the coverage from all surveys involved in mapping Galway Bay for the INFOMAR project. (Click image for more detailed map).

Bathymetry (Water Depth)

Bathymetry of Galway Bay. (Click image for more detailed map)

In general terms, to the east of Black Head water depths remain shallow with depths shallower than 20 metres. Indeed a large portion of the inner bay was suitable for lidar survey which is only typically effective in water depths of 15 metres or less in Irish waters. The bay gradually deepens to the west, with two distinctive areas of deep water between the Aran Islands and the mainland with water depths around 60 metres between Inishmore and Connemara, Co. Galway (the North Sound) and 50 metres between Inisheer and Co. Clare. There are also channels of deeper water between the islands themselves known as Gregory's Sound (between Inishmore and Inishmaan) and Foul Sound (between Inishmaan and Inisheer) where water depths around 30 metres were recorded in both.

Download more detailed bathymetric charts in .pdf format available at various scales here

Shaded Relief

The 3D appearance is achieved using software called Fledermaus. By using some vertical exaggeration, artificial sun-shading (usually as if there is a light source in the nw 315°) and colouring the depths using various colour maps, it is possible to highlight the subtle relief of the seabed. This helps us to quickly understand the variation in depths.

A shaded relief image of rock outcrops on the seabed north of Inishmore. The shallow area shown in red is known as the Brocklinmore Bank. The horizontal layers and jointed appearance of the rocks suggest that it is Carboniferous limestone, the same as that of the Burren and the Aran Islands. (Click image for more detailed map)

A shaded relief image of the seabed between Inishmore and Inishmaan showing a series of dynamic sedimentary structures including four large sand waves of which the largest measures 700 metres along the crest and 100 metres across. Northeast of Inishmore a series of smaller scale sedimentary structures are also visible. These origins of these bedforms may be formed by currents caused by the energetic movement of tides between the islands. (Click image for more detailed map)

The image above shows limestone bedrock and flat featureless seabed southwest of Inishmore. The bedrock is located within 4 km of the island shore. Boulders are evident closer to the shore on inspection in high resolution. (Click image for more detailed map)

The shaded relief image above of the area around Inisheer Island. It shows gently dipping, faulted bedrock outcropping close to the shore. Further offshore the bedrock disappears beneath the seabed.

The offshore Galway area shaded relief data indicates a mixture of soft sediment and outcropping bedrock. The bedrock is characterised by dipping beds that have undergone brittle and ductile deformation. Numerous northeast – southwest orientated faults are found in the area. Several of these faults have been digitised in the image above. Folding of the bedrock is also evident in the image.

Download more detailed shaded relief charts in .pdf format available at various scales here

Backscatter

Multibeam Systems also collect additional information, including the strength of the acoustic signal (or return) from the seafloor. This is known as Backscatter. Differing seafloor types, such as mud, sand, gravel and rock will have different Backscatter values depending on the amount of energy they return to the sonar head. Rocky areas will typically have high returns while soft sediments like mud are more likely to absorb energy and have low Backscatter returns. These differing values are used to generate a grey-order image (i.e. dark for high returns, bright for low returns) of the seabed which can be used to examine the nature of the seafloor.

Image (left) of a MBES backscatter dataset where varying the various shaded of grey represent the intensity of reflected sound recorded by the multibeam sensors. The black areas are rock outcrops, with the darker grey areas around the outcrops and the lines that sweep off to the west probably represent coarser sediments which have been exposed or winnowed of fine sediments (light grey) due to tidal currents. The same image draped over the shaded relief image (right) shows more clearly the rock outcrops and the areas in dark green where sediments have been scoured and built up in the yellow, shallower areas. (Click image for more detailed map)

Backscatter mosaic for CV1402 for the Galway area is shown above. The offshore data block shows a sharp contrast between high reflectivity areas of bedrock outcrop and low reflectivity areas of unconsolidated sediment. The inshore block shows more subtle shades of grey which is interpreted as corresponding to different grain sizes of unconsolidated sediment.

Download more detailed backscatter charts in .pdf format available at various scales here

Sub Bottom Profiler

Used for shallow sub-surface data acquisition. Operating at a lower frequency (3.5 kHz) than the MBES, the energy from the SES Probe 500 pinger is both reflected from and penetrates through the seabed. The sound that penetrates through the seafloor may be reflected due to density changes within the sediments. The result is a series of sound waves returning to the vessel at slightly different times depending on how deep they penetrated through the sediment before returning. These are displayed in the pinger output as a series of layers than can be interpreted to reveal past sedimentation patterns for the area. Penetration and so data quality of the pinger is dependent on sediment type (good through sands, poor through gravels and bedrock) and gas content (poor through gaseous sediments).

The image above is a SBP line from the inner part of Galway Bay. The image spans 9 km from southwest to northeast. Horizontal scale lines are 10 m apart. The top of bedrock is clearly visible as the base reflector over most of the area. Bedrock varies between approximately 6 and 20 m beneath seabed. It is gently dipping. The bedrock is overlain by a soft layer of sediment. The seabed shoals up toward the northeast.

The image above is a SBP line from the offshore area, south west of Inishmore. The image spans 11.2 km from southwest to northeast. Horizontal scale lines are 10 m apart. Bedrock outcrops to the northeast where it is moderately to steeply dipping. The bedrock dips beneath the seabed further to the southwest. A weak, sub-parallel to seabed reflector is evident where bedrock does not outcrop. It is between 5 and 8 m beneath seabed. Another sub-parallel to seabed reflector is found between 3 and 5 m beneath seabed. Both these reflectors onlap the bedrock. Backscatter indicates that the topmost unit is soft sediment.

The image above is an interpretation of crossline 173 at the Ocean Energy Test Site near Spiddal with geological units and reflectors annotated. The LHS is the southern end of the line and the RHS is the northern end. The bedrock is close to the seabed in the north but at depth in the south. Unit 2 attains its maximum thickness at the southern end of the line and pinches out against the bedrock further north. Unit 3 attains its maximum thickness in the south. The survey line is 1 km in length from south to north. As an indication of vertical scale, Horizon 1 is approximately 10 metres beneath the seabed.

Ground Truthing/Seabed Sampling

A range of seabed sampling has been undertaken in Galway Bay, both historically and under the INFOMAR project. These include grab samples, vibrocores and video footage. The locations of the grab samples have been mainly determined by the seabed classification made from the multibeam data.

Seabed sampling locations from Galway Bay where Van Veen grab and Vibrocore instruments were used to ground truth INFOMAR datasets. (Click image for more detailed map)

In 2007, vibrocores were taken using the newly acquired GeoCore 3000/6000 in ideal conditions in Galway Bay. During the Celtic Voyager survey in the same year, 12 grab samples were recovered.

Using the RV Geo in 2008, 24 grab samples were recovered from the inner bay, primarily along the south coast. These are to be followed by further sampling by the Celtic Voyager in 2009 to gain sufficient samples to completely supervise the acoustic seabed classification.

Images of the vibrocores deployment (left) and the successful recovery of a core by INFOMAR staff in Galway Bay (right).

Seabed Classification

Seabed classification is an advanced process in which a backscatter image is interrogated using special software in order to divide it into areas which have similar characteristics. Areas which have a similar nature are grouped together into classes. These classes are colour coded and developed into seabed classification charts. The classes on the chart are then physically sampled in order to ground truth the classification. This allows INFOMAR to match different sediment types with their corresponding colour class thereby producing classified geological maps of the seafloor.

The classification of the multibeam dataset from Galway Bay resulted in the creation of a 5 class classification divided into two types of rock, reflecting the different textures observed from rock outcrops in the bay. Three more classes divided the sediments into Gravels and Coarse Sand, Coarse to Medium Sand and Fine Sand to Mud.

Seabed Classification Chart for Galway Bay.(Click on image for more detailed map)

Further information about the classification process can be found in the Data Processing section.

Download more detailed seabed classification charts in .pdf format available at various scales here

You can download the Seabed Classification in ArcGIS GRID format data from IWDDS

Seabed Habitats \ Seabed Substrate

The seabed substrate information has been derived from a combination of the analysis of geophysical data (multibeam echosounder bathymetry and backscatter) and groundtruthing data acquired as part of the INSS and INFOMAR seabed mapping programmes.

Geology

A major feature in the geology of Galway Bay is the transition from the limestone of the Aran Islands and the Burren, Co. Clare to the granite of west Galway and Connemara. The rock outcrops in the seabed data around the Aran Islands and along the Connemara coast have strikingly different appearances, with the limestones of the islands showing regular layers and the granite outcrops of the north of the bay having a much craggier and irregular nature. In this map the boundary between the two rock types is interpreted as a fault line.

Shipwreck

A 36m long, 9m wide shipwreck was discovered during the Celtic Voyager survey in 2014. It lies in 64m water depth. This wreck is intact. No debris field is evident. It is orientated 126/306°. It has not previously been charted. A scour is present around the entire wreck.

To view the shipwrecks in Google and to download Shipwreck Information Sheets please click here.

3D Movie Flythrough

Click play below to view a 3D movie flythrough of the seabed of Galway Bay. The changing colours represent changes in water depth with red and orange for shallow water, yellow and green for intermediate depths and light and dark blue for deeper water. The flythrough begins with an overview, moves towards the eastern end of the bay before plunging beneath the surface and travelling along the south coast of Galway out to Lettermullan. It then rotates to the southeast and along the margin of the Aran Islands, over banks and patches before emerging just off the coast of Clare, near Doolin before panning back to an overview to finish.

The INFOMAR project acknowledges and thanks the United Kingdom Hydrographic Office (UKHO) for allowing the Admiralty Chart for Galway Bay to be used as a reference map for the area.